The present invention relates to disk circuits and, more particularly, to a method and apparatus for reading information from a magnetic disk.
Conventional magnetic storage devices include a magnetic transducer or xe2x80x9cheadxe2x80x9d suspended in close proximity to a recording medium, for example a magnetic disk, having a plurality of concentric tracks. The transducer is supported by an air-bearing slider mounted to a flexible suspension. The suspension, in turn, is attached to a positioning actuator. During normal operation, relative motion is provided between the head and the recording medium as the actuator dynamically positions the head over the desired track. The relative movement provides an air flow along the surface of the slider facing the medium, creating a lifting force. The lifting force is counterbalanced by a predetermined suspension load so that the slider is supported on a cushion of air, Air flow enters the xe2x80x9cleadingxe2x80x9d end of the slider and exits from the xe2x80x9ctrailingxe2x80x9d end. This air is used to prevent the head from contacting the disk, resulting in damage.
Writing data is typically performed by applying a current to the coil of the head so that a magnetic field is induced in an adjacent magnetic permeable core, with the core transmitting a magnetic signal across any spacing and protecting coating of the disk to magnetize a small pattern or digital bit of the medium within the disk. Reading of the information in the disk is performed by sensing the change in magnetic field of the core as the transducer passes over the bits in the disk. The changing magnetic field induces a voltage or current in the inductively coupled coil. Alternatively, reading of the information may be accomplished by employing a magneto-resistive (MR) sensor, which has a resistance that varies as a function of the magnetic field adjacent to the sensor. In order to increase the amplitude and resolution in reading the bits, the MR sensor is typically positioned on the slider as close to the disk as possible. Connected to these heads or sensors are read circuits which amplify the recorded data and eliminate noise. However, recently, some manufacturers of these MR sensors have switched from MR heads which employ a constant current source to MR heads which employ a constant voltage source.
A typical pre-amplifier for reading information from the read head includes a differential pair of output signals corresponding to reading information from each side of the read head. A problem with this differential output signal is the offset between the differential output signal. Importantly, this offset is directly related to the write/read time which is the time to transfer from a write state to a read state. More particularly, if the offset is great, the write to read time is long and if the offset is small the write to read time is small. Consequently, it has been necessary to provide a circuit to compensate for this offset.
FIG. 2 illustrates such an offset compensation circuit where a current mirror circuit 202 provides current to the base of transistor 226. The collector to emitter current of transistor 226 which is controlled by this base current is the current through the MR head 228. The voltage across resistor 121 is compared with the voltage across resistor 224 by transconductance device 230 and the current output from transconductance device 230 is used to form a voltage on capacitor 232. Since GM is equal to IE divided by VIN, and since the capacitor 232 is typically large because it was external to the chip, GM could be large to result in small offset voltages, namely VIN.
However, since it is desirable to have capacitor 232 move on chip, this requires a smaller capacitor 232 and consequently GM should be correspondingly smaller. However, a small GM dictates that the offset voltage will be large in order to satisfy the above equation. Larger offset voltages as discussed above increases the write to read time and is correspondingly undesirable. Thus, there is a need for a write to read switching time trim circuit which will take into consideration the effect of placing the above-mentioned capacitor on chip and the corresponding effects of having the capacitor a relatively-small size. It is important to maintain a small write to read time, and as a consequence, it is important to maintain a minimum offset between the differential output signals.
The present invention provides a read to write switching time trim circuit that minimizes offset between differential outputs. More particularly, the write to read switching time trim circuit of the present invention minimizes the read to write time required when a hard disk drive system is switching between write mode to a read mode.